1. Technical Field of the Invention
The present invention relates broadly to the field of biomedical transducers and more particularly, to a passive sensor monitoring the heart of a fetus.
2. Related Art and Problem to be Solved
Conventional acoustic fetal heart monitors are described in U.S. Pat. Nos. 4,122,843, 4,299,234, 4,672,976, 4,781,200, 5,140,992 and 5,524,631. U.S. Pat. No. 4,781,200 describes an ambulatory non-invasive automatic fetal monitoring system to detect signals in the 50-110 HZ frequency band. U.S. Pat. No. 4,299,234 describes a fetal heart rate apparatus that simultaneously processes both electro-cardiographic and mechanical cardiographic signals, but does not specifically mention a frequency bandwidth. U.S. Pat. No. 4,672,976 describes a heart sound sensor that utilizes a hydrophone assembly to detect signals in the 10 Hz-2 kHz frequency band. U.S. Pat. No. 4,122,843 describes an electrode system for a heart rate monitor but does not specifically mention a frequency bandwidth.
Examination of these aforementioned patents reveals that, although a particular frequency bandwidth may be specified, none of these patents recognize that there are different frequency bands in which acoustic cardiac signals are transmitted, depending upon the position of the fetus with respect to the maternal abdominal wall. For example, the frequency bandwidth specification of 50-110 Hz disclosed in U.S. Pat. No. 4,781,200 completely excludes the 16-32 Hz band which contains the bulk of the fetal heart energy. In another example, U.S. Pat. No. 4,299,234 discloses a specified frequency bandwidth of 10 Hz-2 kHz bandwidth. Such a bandwidth includes signals from all modes of transmission and provides no method of discrimination.
Successful monitoring of fetal heart activity depends upon the capability of a fetal monitoring system to detect and process acoustic signals produced by the fetus"" heart no matter how the fetus is positioned with respect to the maternal abdominal surface. Such a capability has not been addressed by prior art or conventional devices, methods and techniques.
What is needed is a fetal heart monitoring system that can detect acoustic signals emanating from the heart of the fetus with regard to the position of the fetus with respect to the maternal abdominal surface. In particular, what is needed is a fetal heart monitoring system that can detect acoustic signals emanating from the heart of the fetus whether or not the fetus is in direct contact with the maternal abdominal wall.
It is therefore an object of the present invention to provide a fetal heart monitoring system that fulfills the aforementioned needs and to address the occasional inefficacy of prior art or conventional devices.
Other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to a fetal heart monitoring system for detecting and processing acoustic fetal heart signals transmitted by different signal transmission modes. One signal transmission mode, the direct-contact mode, occurs in a first frequency band when the fetus is in direct contact with the maternal abdominal wall. Another signal transmission mode, the fluid propagation mode, occurs in a second frequency band when the fetus is in a recessed position with no direct contact with the maternal abdominal wall. The second frequency band is relatively higher than the first frequency band. The fetal heart monitoring system of the present invention detects and processes acoustic fetal heart signals that are in the first frequency band and second frequency band.
Accordingly, the present invention is directed to, in one aspect, a fetal heart monitoring system, comprising a passive fetal heart monitoring sensor having a plurality of sensor elements for acquiring acoustic signals emitted from a fetus inside a body and outputting a plurality of sensor signals, a selection circuit for selecting a particular one of the sensor signals, a signal processing device having a first signal processing channel for processing acoustic signals in a first frequency band and a second signal processing channel for processing acoustic signals in a second frequency band. The signal processing device has a first state such that the signal processing device outputs sensor signals processed by the first signal processing channel and a second state such that the signal processing device outputs sensor signals processed by the second signal processing channel. The fetal heart monitoring system further includes a monitoring device responsive to the signal processing device for monitoring the characteristics of the processed signals outputted by the signal processing device and determining if such characteristics meet or exceed predetermined criteria, and a control device for configuring the signal processing device to the first state so as to process selected sensor signals with the first signal processing channel if such processed signals meet or exceed the predetermined criteria and for configuring the signal processing device to the second state so as to process selected sensor signals with the second signal processing channel if the sensor signals processed by the first signal processing channel do not meet the predetermined criteria.
The control device configures the signal processing device back to the first state so as to process selected sensor signals with the first signal processing channel if the signals processed by the second signal processing channel do not meet predetermined criteria.
In another aspect of the present invention, the monitoring device can include additional signal filtering capability.
In another aspect, the present invention is directed to a method of monitoring fetal heart activity, comprising the steps of (a) providing a fetal heart monitoring system comprising a passive fetal heart monitoring sensor having a plurality of sensor elements for acquiring acoustic signals emitted from a fetus inside a body and outputting a plurality of sensor signals, and a signal processing device having a first signal processing channel for processing acoustic signals in a first frequency band and a second signal processing channel for processing acoustic signals in a second frequency band, the signal processing device having a first state such that the signal processing device processes sensor signals with the first signal processing channel when such processed signals meet predetermined criteria and a second state such that the sensor signals are processed by the second signal processing channel when the processed signals outputted by the first signal processing channel do not meet predetermined criteria, (b) processing the sensor signals with one of the signal processing channels of the signal processing device, (c) monitoring the characteristics of the processed sensor signals to determine if such processed sensor signals meet predetermined criteria, and (d) configuring the signal processing device so as to process the sensor signals with the other signal processing channel if the processed signals do not meet predetermined criteria.
In one aspect of the method, the processing step comprises the steps of configuring the signal processing device in the first state, filtering the selected sensor signals with a low pass anti-aliasing filter, and filtering the previously filtered selected sensor signals with a high pass filter configured to pass only signals having frequencies in the first frequency band.
In another aspect of the method, the processing step comprises the steps of configuring the signal processing device in the second state, filtering the selected sensor signals with a low pass anti-aliasing filter, filtering the previously filtered selected sensor signals with a high pass filter configured to pass only signals having frequencies in the second frequency band, and thereafter, amplifying the filtered signals.
In yet another possible aspect of the method, the step of monitoring the characteristics of the processed sensor signals to determine if such processed sensor signals meet predetermined criteria comprises additional filtering of the processed sensor signals.